The present invention relates to a switching arrangement for the control of the electric motor of a hydraulic pump which forms part of the auxiliary pressure supply system of an automotive brake system. The arrangement includes a pressure switch which closes a switching contact in the feed circuit of a motor relay whose operating contact switches the pump motor if the auxiliary pressure drops to a lower limit and opens if the auxiliary pressure rises to an upper limit. An additional switching contact is provided which responds in order to release a warning signal if the auxiliary pressure falls short of the pressure minimum and opens if the auxiliary pressure rises to a limit which is below the switching thresholds of the pressure switch.
A known brake system with a hydraulic booster and an anti-lock control system of this type has an auxiliary pressure supply system which comprises a hydraulic pump with electric motor drive and a pressure accumulator. In operation, the auxiliary pressure can vary within given limits of, for example, within 140 and 180 bar. The hydraulic pump is switched on as soon as the auxiliary pressure drops to the lower limit and remains in operation until the upper limit is reached. If, for example, due to a defect, the auxiliary pressure drops below the lower limit and reaches an auxiliary pressure minimum, which is set to 105 bar in the known brake system, a warning signal is provided. A brake warning lamp is switched on and the anti-lock control system is, if required, locked in part or completely so that the functioning of the brake without anti-lock control is not jeopardized. The warning signal is only discontinued and the locking brought to an end when the pressure has risen to a specific limit beyond the pressure minimum, with the limit still being below the lower limit of the operating range of the auxiliary pressure.
In the known brake system, the pump motor is switched on and off by means of the operating contact of a relay, a so-called motor relay. The exciting winding of the relay is also switched by means of a mechanical contact of a pressure switch. Mechanical switches of this type are comparatively susceptible to wear. The contacts can weld together or burn down in the event of specific defects within the system since in that type of systems, as is generally known, relatively high cut-in and cut-off currents can occur. In pump motors of this type in-rush currents of up to 50 ampere can be measured. The energies accumulated in the relay coil and in the motor windings produce high cut-off spikes in the respective branches. Furthermore, some contact defects only become apparent when the accumulator pressure is used up or after the continuously running pump motor has become defective.